This invention relates to a process of preparing polycarbonate resins in organic solutions wherein phosgene is added to a dihydric phenol and wherein the stoichiometric end point is accurately controlled by using 4-(p-nitrobenzyl) pyridine as a chromophore to indicate the end point.
It is known from U.S. Pat. No. 3,144,432 dated Aug. 11, 1964, that polycarbonate resins can be prepared by reacting phosgene and bis A in a combined pyridine-halogenated hydrocarbon solvent.
It is known from Cockerill, et al, Tetrahedron Letters 1:27-28 (1972) that the reaction of phosgene with 4-(p-nitrobenyl) pyridine produces a color body and the pyridine compound has been used for the estimation of microgram levels of phosgene.
It is also known from U.S. Pat. No. 3,641,025 dated Feb. 8, 1972 that the 1:1 stoichiometry between reactions of acid chlorides such as phosgene and amines or alcohols or thio alcohols can be controlled or monitored by adding a fluorescent amine to samples of the reaction mixture. However, this method has the disadvantage that fluorometric analysis equipment is required.